Fentanyl is a synthetic narcotic analgesic commonly used in anesthesia. Little is known about how critically ill patients metabolize and eliminate fentanyl. 20 patients will be studied over a 3 year period to determine the pharmacokinetics. Quantitative analysis of plasma samples will be undertaken in the CPRL. The method of analysis is a gas chromatographic system which is both sensitive and selective. Data analysis will be done in the same laboratory.